1. Field of the Invention
The invention relates generally to zinc binding proteins, and more specifically to a zinc finger protein that is expressed in plant meristem and has the characteristics of a meristem specific transcription factor, to methods of using the zinc finger protein to regulate gene expression and to modulate plant growth and development, and to a transgenic plant containing in its genome a transgene, which includes a polynucleotide encoding the zinc finger protein, or a nucleotide sequence complementary to the encoding polynucleotide.
2. Background Information
Technologic developments continually advance in an effort to address the need to increase plant yield in order to feed the expanding world population. Biotechnology is playing an increasingly important role in this effort by providing, for example, plants having increased resistance to drought and insect infestation. For many plants such as corn, rice, and soybean, seed provides the source of food products, including grain, and can be eaten directly or processed into flour, milk products, and the like. For other plants, edible seeds, roots, stems, leaves, bulbs and tubers provide a source of vegetables. Fruits, which are the ripened reproductive body of plants, also are an important food source.
Because many foods are derived, either directly or indirectly, as a result of plant flowering, methods for increasing flowering efficiency and numbers of flowers produced of plants can result in increased yield. An important aspect of plant flowering relates to the regulation of meristem activity in the production of florets. As such, an understanding of the factors that influence the activity of apical, inflorescence, and floral meristems can provide tools for manipulating the flowering characteristics of a plant. Further, while providing a means to increase yield of crop plants, such tools also can be useful in the ornamental plant industry, providing, for example, a means to increase the number and/or size of flowers produced by a plant.
Genes that regulate floral organ formation have been identified, including, for example, genes that control the establishment of floral organ identity, and the products of many of these genes have been identified as transcription factors, or as putative transcription factors based on their structural features. Evidence indicates that gene expression leading to the floral organ formation is regulated, at least in part, at the transcriptional level by a hierarchical expression of homeotic genes, including transcription factors. As such, many of the target genes of the homeotic genes can encode transcription factors that, in turn, regulate downstream events, thus providing specialized roles in flower development.
Zinc finger proteins containing two conserved cysteine residues and two conserved histidine residues (C2H2 zinc finger proteins) comprise a family of transcription factors that have homeotic functions, including, for example, Kruppel and Hunchback, which are involved in transcriptional control of development in Drosophila. In plants, the ZFP-2 family of transcription factors may represent a class of transcription factors that control developmental processes.
Zinc finger domains comprise a portion of zinc finger proteins that specifically bind target DNA sequences (e.g., gene promoters). Since the initial description of zinc finger domains in the Xenopus transcription factor TFIIIA, the domains have been identified as a common feature of many nucleic acid binding proteins. Zinc finger domains generally are about 25 to 30 amino acid residues in length, and contain the conserved C2H2 motif (C—X2-C—X12-H—X3-H (SEQ ID NO:27), where X is any amino acid residue and the number indicates the number of X residues). Further, the X residues generally are not random; for example, the 12 residues between the second Cys and the first H is generally are polar and basic, and implicate this region as directly involved in nucleic acid binding. Zinc ion is a crucial component of the zinc finger domain tertiary structure. All zinc finger domains bind 1 atom of zinc in a tetrahedral array, yielding a finger like projection that can interact with nucleotides in the major groove of a nucleic acid molecule.
The identification of transcription factors involved in floral organ identity provides a means to manipulate whether a plant flowers or does not flower. However, such factors do not allow one to manipulate the number of flowers or other organs on a plant and, therefore, are limited in their use for increasing plant yield. Thus, a need exists for compositions and methods that can affect plant tissue determination such as the zinc finger protein of the present invention that is expressed in plant meristem.
Nearly all crops may be benefited by the manipulation of growth and development characteristics. As such, mutations in the reception and signal transduction of gibberellins leading to dwarf-like plants have been described as advantageous in many crop plants (U.S. Pat. No. 6,307,126; U.S. Pat. No. 6,762,348; U.S. Pat. No. 6,830,930; U.S. Pat. No. 6,794,560). This was especially true in high-yielding, semi-dwarf wheat varieties where the reduced plant stature was most advantageous in increasing grain production per plant and superior straw strength. The shorter, stronger straw greatly reduces the losses resulting from lodging or flattening of the standing wheat plants by rain and high winds. In addition a concomitant increase in harvest index was evident shifting more photoassimilates from vegetative growth components to the grain.
Specific genes that regulate the expression of GA biosynthetic pathway genes are the subject of continued research. Much work has been published on the genes involved in sensing GA or transducing the GA signal downstream to affect plant growth and development. One known direct effect on controlling the expression of a biosynthetic enzyme in the GA pathway involves a KNOX-like homeodomain containing a transcription factor. In tobacco, the KNOX-like NTH15, was shown to repress the GA-20 oxidase gene, Ntc12, in shoot apical meristems as a mechanism to repress aspects of cellular differentiation in the meristem (Sakamoto, T., et al., KNOX homeodomain protein directly suppresses the expression of a gibberellin biosynthetic gene in the tobacco shoot apical meristem. Genes Dev, 2001. 15: 581-90). Evidence gathered indicates that the ZmZFP2 gene affects plant growth and development in a similar manner. Ectopic expression of ZmZFP2 in maize reduces plant stature (markedly in more strongly expressed lines) while inhibition of ZmZFP2 gene by RNAi enhances plant height in a statistically significant manner.